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Let's first cover the basics. What is Tire pressure? Tire pressure is the manufacturer's recommended pressure for a tire, dependent on load, speed, etc., usually given in bar or pound-force per square inch (psi). The proper tire pressure reading is COLD inflation pressure and is the inflation pressure of tires before the car is driven and the tires warmed up due to the fact that air expands as it heats up. Tire pressure readings taken when a tire is warm would be not be accurate.

UNDER inflation as well as OVER inflation of your tires will results in unnecessary tire stress, irregular wear, loss of control and accidents.  A tire's inflation pressure cannot be accurately determined through visual inspection.  A tire can lose up to half of its inflation pressure and not appear to be flat!  A tire’s tread life and structural integrity can be drastically reduced due to under inflation and if you've purchased a set of NEW tires recently, you know they aren't cheap. 

Low tire pressure will also allow the tire to bend more as it rolls. This will build up internal heat, increase rolling resistance and cause a noticeable reduction in fuel economy. If you've bought a tank of gas lately, you know gas isn't cheap. Tires do not only leak air if punctured, they also leak air naturally, and over a year, even a typical new, properly mounted tire can lose from 3 to 9 psi (pounds per square inch), roughly 10% or even more of its proper pressure.

Low Tire Pressure or Under Inflation
Low tire pressure reduces the vehicle’s stability even under the best of conditions, but it also makes it much more difficult for you to maintain control in bad weather or during emergency maneuvers, such as when someone texting on the cell phone swerves into your lane or a deer, hanging out along the freeway, decides to jump out of the woods in front of you.

Problems also can result from worn tires, especially those that have gone bald or have damage to the tire that could lead to a catastrophic blow-out.

an addition to the potential problems listed above, an underinflated tire can't maintain its shape and becomes flatter than the manufacturer intended while it rolls along in contact with the road. If your vehicle’s tires are underinflated by only 5 psi (pounds per square inch) it could lead to tire failure. You can also experience a loss of steering precision and cornering stability. While 5 psi doesn’t seem excessively low, remember, 5 psi is usually about 20% of the tire’s recommended pressure.

Too Much Pressure or Over Inflation
An over inflated tire is stiff and the ride will be rough. They don't give in much to bad road conditions and the size of its footprint in contact with the road is less than what the tire manufacturer intended when they designed the tire. If a vehicle's tires are overinflated by 5 psi, they could be more easily be damaged when running over that pothole you didn't see or the debris in the road you didn't see. On the other hand, higher tire pressures usually provide an improvement in cornering and steering response.  Unless you are going to be in a road race sometime soon you might consider keeping your tire pressure at the optimal number.

How and When to check your tire pressure:
Search your vehicle for accurate information about proper tire for your ride. It will be somewhere in the car. Cold inflation pressure is the inflation pressure of tires before the car is driven and the tires warmed up. Recommended cold inflation pressure is displayed on the owner's manual and on the placard (or sticker) attached to the vehicle door edge, pillar, glovebox door or fuel filler flap. Sometimes, there will be a range specified, or two different recommendations, depending on load. Anywhere between these numbers should be safe.

Changing the tire size or going to a plus-size fitment will change the required inflation pressure somewhat. Within the range of optional OE (Original Equipment) tire sizes, the vehicle manufacturer's recommendations should be followed. For sizes outside the OE range, your tire dealer should be able to help. Many tire manufacturers includes pressure recommendations for stock and plus-sized tires, by vehicle, which your tire dealer should have access to. Typically, these are 0-3 psi higher for a Plus-one or Plus-two fitment. Also listed are maximum load ratings at maximum inflation pressure for each size of each tire.

DO NOT inflate tires to the pressure stamped on the sidewall of your tires.  That’s the MAXIMUM pressure that your tire is designed. If you're even thinking of using a greater pressure than that stamped on the sidewall, then you need to use a tire designed for a pressure which exceeds the amount you wish to use.

Check your tire pressure in the morning  before you’ve driven even one mile or check it at least three hours after the vehicle has been stopped.- Air expands when it’s heated and contracts when it cools down. Even in cold weather, the air in a tire becomes warmer and pressure increases as the tire is driven.

If you have to drive any distance to get air, check and record the tire pressure first and add the appropriate amount of air when you get to a pump.  Remember, many gauges on convenience stores and gas stations have been severely abused and are usually inaccurate. You should always have a quality air pressure gauge living in your glove box. Check your tire pressure at least once a month and always before long trips

If you overfill the tire, you can release air by pushing on the metal stem in the center of the valve stem with your fingernail or the tip of a pen. Then recheck the tire pressure. Never "bleed" or reduce air pressure when your tires are hot.

Replace the valve cap. If you don't have a valve cap, get one.  A high-quality valve cap can help maintain the tire’s air pressure if the valve core fails for any reason. Valve caps also keep out moisture, which could freeze and in turn depress the valve core, causing air loss. The cap also keeps out dust and dirt particles, which could also interfere with the proper operation of the valve core and cause air loss.

Remember to check the spare (Note: some spare tires require a higher inflation pressure).
 

Tire Pressure Monitoring Systems. (TPMS)
"I don't need to check my tire pressure, I've got a Tire pressure monitoring system."  WRONG!! 
If you're driving a car, truck or SUV built in the past few years, there's a good chance that it has a TPMS. In fact, starting with all 2008 models, it's a required feature.  In response to the rollover incidents involving the Ford Explorer and Firestone tires, Congress enacted the TREAD Act in 2000. Part of this act got the process moving for having a TPMS in every vehicle.

While tire-pressure monitoring systems are great safety aids, they may allow tire pressure to fall as much as 25% below the recommended level before giving a warning, and some tire experts say such a loss is too great. Losing that much air from a tire can cause handling problems and possibly damage the tire, so it is better to check each tire yourself once a week or so. Also, monitors can malfunction

Bottom line: Don't bet all the marbles on your TPMS. Once you understand (do a Google search) how your TPMS works, you'll understand why it doesn't eliminate the need to check your tire pressure regularly.